It is desirable that a communication apparatus such as a mobile phone or a wireless LAN should operate with a low power consumption while ensuring the accuracy of a transmission signal, regardless of the magnitude of the output power level. Thus, to the communication apparatus, a transmission circuit using polar modulation is applied that is small in size, operates at a high efficiency, and outputs a transmission signal having a high linearity.
FIG. 12 is a diagram showing a conventional transmission circuit 900 using polar modulation. In FIG. 12, the conventional transmission circuit 900 using polar modulation includes a radio frequency integrated circuit (RF-IC) 910, a control voltage supply section 920, and a power amplifier (PA) 930. Further, the radio frequency integrated circuit 910 includes a signal generation section 911, a DA converter (DAC) 912, a phase modulator (PM) 913, and a variable gain adjustment section (VGA) 914.
An input signal input to the signal generation section 911 is decomposed into an amplitude signal M and a phase signal P. Between the two signals, the amplitude signal M is input to the control voltage supply section 920 via the DA converter 912. The control voltage supply section 920 generates a control voltage Vcc on the basis of the input amplitude signal, and supplies the control voltage Vcc to the power supply terminal of the power amplifier 930. The phase signal P is phase-modulated by the phase modulator 913, and is input to an input terminal of the power amplifier 930 via the variable gain adjustment section 914. The power amplifier 930 amplitude-modulates, on the basis of the control voltage Vcc supplied from the control voltage supply section 920, the phase signal input from the variable gain adjustment section 914, and outputs the resulting signal as a transmission signal.
In a high-output region, the conventional transmission circuit 900 using polar modulation operates in a high-output mode, and causes the power amplifier 930 to operate in a saturation region. Specifically, the input power level of the phase signal to be input from the variable gain adjustment section 914 to the power amplifier 930 is fixed, and the phase signal having the fixed input power level is amplitude-modulated on the basis of the control voltage Vcc from the control voltage supply section 920.
In a low-output region, on the other hand, the conventional transmission circuit 900 using polar modulation operates in a low-output mode, and causes the power amplifier 930 to operate in a linear region. Specifically, the phase signal to be input to the power amplifier 930 via the variable gain adjustment section 914 is amplitude-modulated on the basis of the control voltage Vcc from the control voltage supply section 920.
Here, for example, the UMTS (Universal Mobile Telephone System) requires an increase in the bandwidth of a transmission signal. This requires an increase in the dynamic range of the power amplifier 930, using the high-output region and the low-output region together.